When directional light enters a water surface, so-called caustic patterns are projected on the bottom. Such projected patterns occur when the light is highly collimated (such as light from the sun or a from a narrow beam light source) or when the source is very small such as an LED. Large diffuse sources, like conventional tungsten tubes, or indirect lighting, do not produce caustics.
In aquarium lighting, LEDs have the advantage of high efficacy and easy spectral tunability. However, the resulting array of point-like sources may result in a restlessly moving caustic pattern in and around the aquarium, as well as colour patterns and coloured shadows.
While the caustic patterns to some extent may be appreciated inside the aquarium (since it is caused by the water dynamics), part of the light may also leak out of the aquarium and create a restless pattern on the floor in the vicinity of the aquarium. This may be found to be a disturbing effect. It occurs when light enters the water at a steep angle to the vertical normal (in fact above 63 degrees to the vertical). This effect is illustrated in FIG. 1, in which beam 10 has a steep angle to the vertical above 63 degrees, whereas beam 12 has a shallower angle below 63 degrees.
Water has a relatively low refractive index (typically n=1.34), and this means that light at these high incident angles is not reflected by total internal reflection at the side wall, but it can refract out of the water volume and land on the floor. This light which has leaked from the sides of the aquarium may then project restless caustic patterns onto the floor.
Light 12 at smaller angles stays within the aquarium by total internal reflection at the sides of the aquarium.
A solution to keep all light within the water volume is to limit the light to beam angles well within 63 degrees to the vertical direction, for example by placing a collimator on each LED. While this directional lighting is more efficient since more light stays inside the aquarium, it also has some drawbacks. Directional lighting, in particular with hard cut off angles, produces so-called scalloping on the back side of the aquarium, which is visible as alternating bright and dark areas, as well poor illumination of the objects in the aquarium. In particular, a high contrast between the top and bottom parts of an object can result, with loss of visible shape details. Furthermore, homogenous lighting is recognized as benefiting coral growth, such as the speed of growth. Directional light from above may for example result in bleaching of the bottom parts of the coral, which is visible when viewing the aquarium from the front.
U.S. Pat. No. 8,646,934 discloses an aquarium lighting system in which lenses are used to define precise angular outputs from lighting elements.
There is therefore a need for an aquarium lighting solution which is efficient and versatile, for example enabling implementation using LEDs, and which gives a minimum of disturbing caustic patterns on the floor, as well as controlling the caustic pattern (e.g. the frequency) in the aquarium. Scalloping or harsh shadows should also be avoided in the aquarium.